82 Observations upon the Habits, Structure and [February, 
the body through which it passes, but it is undoubtedly to be 
considered as a single vessel, since it is of nearly uniform calibre 
throughout its course, and. the rythmical contractions, which are. 
a very marked feature and readily seen, especially in the young, 
pass from behind forwards, at somewhat lengthy intervals, along 
its entire length. Up to quite recently this long tubular vena 
cava was considered to be the sole pulsatile organ of Amphioxus, 
and as a “tubular heart,” which was the term applied to it, was 
an anomaly among vertebrates, it was thought fit to designate the 
“thin” from the “thick” hearted vertebrata, and thus separate 
Amphioxus from the rest of the family by instituting for its sole 
reception the class Leptocardia. But in 1876 Langerhans 
announced! the discovery of the organ, which is described above, 
as being situated at the anterior extremity of the vena cava, and 
which he considers as representing the heart. If this is the case, 
and Amphioxus does indeed possess a thick-walled pulsatile cavity, 
the above classification may have to be revised, but even then the 
pulsatile character of the vena cava, together with the other pecu- 
liarities in the organization of Amphioxus, may be considered of 
sufficient importance to make it the unique representative of a 
distinct class. From each side of the pharyngeal portion of the 
vena cava a series of vessels are given off, which correspond in 
number and position to the main bars of the branchial arches, 
and through which these vessels pass to the dorsal edge of the ` 
pharynx, where they unite into two aorta, which run back with- 
out any intercommunication, one upon either side of the median 
line, to the cesophagus, where they unite into a single tube, which 
proceeds as a distributing vessel to the end of the body. Each 
series of these branchial or blood-renovating vessels forms its 
own aorta, and each individual vessel has, at its base or origin, a 
small dilatation or bulb-like enlargement to which the name of 
“branchial heart” has been given. These “hearts” lie in the — 
alternating triangular spaces between the curved bases of the 
cartilaginous arches, a, Fig. 4, Pl. 1, and probably act as elastic 
reservoirs to render the flow of blood steady and continuous 
through the branchiz. The main heart, into which a certain, 
perhaps a large proportion of blood from the vena cava passes, 
gives off three vessels, two small paer from its anterior border, 
; Anat: a Amphi -us lanceolatus Wa Dr. Paul Langerhans. Archiv 
für  Miksokopische Anatomie. Band 12, 7 336, fig. 49c. Boen, 1876. 
N 
